Technique for Forming a Cathode of a Wet Electrolytic Capacitor

ABSTRACT

A technique for forming a cathode of a wet electrolytic capacitor is provided. The cathode contains a metal substrate having a roughened surface and a conductive coating that contains a substituted polythiophene. The degree of surface contact between the conductive coating and the roughened surface is enhanced in the present invention by selectively controlling the manner in which the conductive coating is formed. More particularly, the conductive coating is formed by applying a precursor solution to the roughened surface that includes both a precursor thiophene monomer and an oxidative catalyst. Contrary to techniques in which either the monomer or catalyst is applied separately and initially contacts the metal surface, the presence of the monomer and catalyst within the same solution allows polymer chains to grow immediately adjacent to the surface of the metal substrate and within the pits. This can significantly increase the degree of contact between the conductive coating and metal substrate, thereby resulting in improved mechanical robustness and electrical performance (e.g., reduced equivalent series resistance and leakage current). To minimize the likelihood of premature polymerization, the oxidative catalyst is also employed in an amount less than what is required to fully react all of the reagent assuming 100% yield (i.e., “stoichiometric amount”). This can slow the polymerization of the monomer, creating oligomers that are shorter than if fully polymerized, which can better penetrate into the roughened regions of the metal substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of U.S. Provisional Application Ser. No. 61/383,445 having a filing date of Sep. 16, 2010.

BACKGROUND OF THE INVENTION

Wet capacitors are increasingly being used in the design of circuits due to their volumetric efficiency, reliability, and process compatibility. Wet capacitors typically have a larger capacitance per unit volume than certain other types of capacitors, making them valuable in high-current, high power and low-frequency electrical circuits. One type of wet capacitor that has been developed is a wet electrolytic capacitor that includes a valve metal anode, a cathode, and a liquid electrolyte. The unit cell voltage in this type of capacitor is generally higher due to the formation of a dielectric metal oxide film over the anode surface. Wet electrolytic capacitors tend to offer a good combination of high capacitance with low leakage current. Another type of wet capacitor is a wet symmetric capacitor in which the anode and cathode are similar in terms of structure and composition. The unit cell voltage in this type of capacitor is generally low due to the inevitable decomposition of the electrolyte at high voltage. Whether electrolytic or symmetric, however, the cathodes of wet capacitors typically include a substrate and a coating that provides high capacitance through a faradic or non-faradic mechanism. Conventional coatings include activated carbon, metal oxides (e.g., ruthenium oxide), and the like. Unfortunately, however, the coatings can become easily detached under certain conditions, such as in the presence of aqueous electrolytes.

As such, a need remains for a high voltage wet electrolytic capacitor that possesses good mechanical robustness and electrical performance.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a method for forming a cathode of a wet electrolytic capacitor is disclosed. The method comprises applying a precursor solution to a roughened surface of a metal substrate, wherein the precursor solution contains a substituted thiophene monomer and an oxidative catalyst and the oxidative catalyst is employed in an amount less than the stoichiometric amount. The substituted thiophene monomer is polymerized to form a coating that contains an intrinsically conductive substituted polythiophene.

Other features and aspects of the present invention are set forth in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof to one skilled in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which:

FIG. 1 is a cross-sectional view of one embodiment of the wet electrolytic capacitor of the present invention; and

FIG. 2 is a cross-sectional view of a micro-roughened metal substrate that may be employed in one embodiment of the present invention.

Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention.

Generally speaking, the present invention is directed to a technique for forming a cathode of a wet electrolytic capacitor. The cathode contains a metal substrate having a roughened surface and a conductive coating that contains a substituted polythiophene. The degree of surface contact between the conductive coating and the roughened surface is enhanced in the present invention by selectively controlling the manner in which the conductive coating is formed. More particularly, the conductive coating is formed by applying a precursor solution to the roughened surface that includes both a precursor thiophene monomer and an oxidative catalyst. Contrary to techniques in which either the monomer or catalyst is applied separately and initially contacts the metal surface, the presence of the monomer and catalyst within the same solution allows polymer chains to grow immediately adjacent to the surface of the metal substrate and within the pits. This can significantly increase the degree of contact between the conductive coating and metal substrate, thereby resulting in improved mechanical robustness and electrical performance (e.g., reduced equivalent series resistance and leakage current). To minimize the likelihood of premature polymerization, the oxidative catalyst is also employed in an amount less than what is required to fully react all of the reagent assuming 100% yield (i.e., “stoichiometric amount”). This can slow the polymerization of the monomer, creating oligomers that are shorter than if fully polymerized, which can better penetrate into the roughened regions of the metal substrate.

Various embodiments of the present invention will now be described in more detail.

I. Cathode

A. Metal Substrate

The metal substrate of the cathode may include any metal, such as tantalum, niobium, aluminum, nickel, hafnium, titanium, copper, silver, steel (e.g., stainless), alloys thereof (e.g., electrically conductive oxides), composites thereof (e.g., metal coated with electrically conductive oxide), and so forth. Titanium and tantalum, as well as alloys thereof, are particularly suitable for use in the present invention. The geometric configuration of the substrate may generally vary as is well known to those skilled in the art, such as in the form of a container, can, foil, sheet, screen, mesh, etc. In one embodiment, for example, the metal substrate forms a casing having a generally cylindrical shape. It should be understood, however, that any geometric configuration may be employed in the present invention, such as D-shaped, rectangular, triangular, prismatic, etc. The casing may optionally include a lid that covers the anode and electrolyte, which may be formed from the same or different material than the casing.

Regardless of its particular form, the substrate is roughened to increase its surface area and increase the degree to which the conductive polymer is able to adhere thereto. In this regard, a variety of techniques may generally be employed to roughen the surface of the metal substrate. In one embodiment, for example, the surface is chemically etched, such as by applying a solution of a corrosive substance (e.g., hydrochloric acid) to the surface. The surface may also be electrochemically etched, such as by applying a voltage to a solution of the corrosive substance so that it undergoes electrolysis. The voltage may be raised to a sufficiently high level to initiate “sparking” at the surface of the substrate, which is believed to create high local surface temperatures sufficient that etches away the substrate. This technique is described in more detail in U.S. Patent Application Publication No. 2010/0142124 to Dreissig, et al., which is incorporated herein in its entirety by reference thereto for all purposes.

In addition to chemical or electrochemical roughening techniques, mechanical roughening may also be employed. In one embodiment, for instance, the surface of the metal substrate may be abrasive blasted by propelling a stream of abrasive media against at least a portion of a surface thereof. Among other things, this physically stresses and deforms the surface to create small pits on the surface so that it becomes pitted and roughened. These pits can increase the degree to which the conductive polymer is able to adhere to the metal substrate. Further, abrasive blasting may distribute the pits in a substantially uniform manner so that the surface is generally smooth on a macroscopic level. The surface area of the substrate may also be increased. For instance, the area of the surface of the substrate prior to roughening may range from about 0.05 to about 5 square centimeters, in some embodiments from about 0.1 to about 3 square centimeters, and in some embodiments, from about 0.5 to about 2 square centimeters. The ratio of the area of the micro-roughened surface to that of the initial surface (prior to micro-roughening) may likewise be from about 1 to about 5, and in embodiments, from about 1.1 to about 3. The increase in surface area can allow for increased cathode capacitance for a given size and/or capacitors with a reduced size for a given capacitance.

Referring to FIG. 2, for example, one embodiment of a metal substrate 200 is shown that has been abrasive blasted to form a micro-roughened surface 204 having a plurality of pits 206. The relative size and spacing of the pits 206 may vary depending on the desired properties for the capacitor. For example, the average depth (“D”) of the pits 206 may be from about 200 to about 2500 nanometers, in some embodiments from about 300 to about 2000 nanometers, and in some embodiments, from about 500 to about 1500 nanometers. Likewise, adjacent pits 206 may be spaced apart a “peak-to-peak” distance (“P”) that ranges from about 20 to about 500 micrometers, in some embodiments from about 30 to about 400 micrometers, in some embodiments, from about 50 to about 200 micrometers. The number of pits 206 may also be high enough to produce the desired increase in surface area. For example, the surface may possess from 1 to 20, in some embodiments, from 2 to 15, and in some embodiments, from 3 to 10 pits per 100 square micrometers. The pits 206 may be disposed uniformly or non-uniformly across the surface 202. For example, the pits 206 may be present in a spaced-apart fashion over the surface so that they form “island-like” structures. It should be understood that the entire surface of the substrate need not be abraded. In fact, in certain embodiments, it may be desired to only abrade a portion of the metal substrate so that the remaining portion is relatively smooth for attaching a sealing mechanism. For example, a portion of the substrate may be covered by a masking device (e.g., ferrule, tape, etc.) during abrasion so that the pits are formed only in the desired locations. When employing a cylindrical substrate, for instance, it may be desirable to use a generally cylindrical, hollow ferrule to mask a top portion of the substrate.

The methods employed to abrasively blast the surface may be selectively controlled to achieve the desired features. Suitable methods may include, for example, sandblasting, bead blasting, pellet blasting, etc. The abrasive media employed in such methods may vary and include, for example, ceramic particles, metal particles, polymeric particles, liquids (e.g., water), etc. Sandblasting is particularly suitable for use in the present invention and generally involves propelling a stream of ceramic media (e.g., silicon carbide, aluminum oxide, titanium dioxide, etc.) through a nozzle and against the surface of the substrate. The size of the abrasive media may be selected based on the type of substrate, the pressure employed, and the desired qualities of the finished substrate. For example, the abrasive media may have an average size of from about 20 micrometers to about 150 micrometers. Further, the pressure and time that the abrasive media is propelled toward the surface may range from about 1 to about 50 pounds per square inch, and in some embodiments from about 10 to about 35 pounds per square inch, for a time period of from about 1 to about 50 seconds, in some embodiments from about 5 to about 40 seconds, and in some embodiments, from about 10 to about 30 seconds. At such conditions, the distance that the injection nozzle is spaced from the surface of the metal substrate may also be controlled to achieve the desired pit formation, such as from about 0.1 to about 5 inches from the surface of the substrate. The nozzle may be stationary or it may be moved relative to the substrate during application of the abrasive media. When blasting the interior surface of a cylindrical casing, for example, the nozzle may be rotated or remain stationary while the casing is rotated. One or more blasting steps may generally be employed. Once complete, any abrasive media remaining on the surface of the metal substrate is typically removed, such as by washing the substrate.

B. Conductive Coating

As indicated above, a conductive coating is formed on the micro-roughened surface of the metal substrate. The conductive coating contains a substituted polythiophene, which is r-conjugated and has intrinsic electrical conductivity (e.g., electrical conductivity of at least about 1 μS cm⁻¹). In one particular embodiment, the substituted polythiophene has recurring units of general formula (I), formula (II), or both:

wherein,

A is an optionally substituted C₁ to C₅ alkylene radical (e.g., methylene, ethylene, n-propylene, n-butylene, n-pentylene, etc.);

R is a linear or branched, optionally substituted C₁ to C₁₈ alkyl radical (e.g., methyl, ethyl, n- or iso-propyl, n-, iso-, sec- or tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1-ethylpropyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl, n-octadecyl, etc.); optionally substituted C₅ to C₁₂ cycloalkyl radical (e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl cyclodecyl, etc.); optionally substituted C₆ to C₁₄ aryl radical (e.g., phenyl, naphthyl, etc.); optionally substituted C₇ to C₁₈ aralkyl radical (e.g., benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-, 3,5-xylyl, mesityl, etc.); optionally substituted C₁ to C₄ hydroxyalkyl radical, or hydroxyl radical; and

x is an integer from 0 to 8, in some embodiments, from 0 to 2, and in some embodiments, x is 0. Example of substituents for the radicals “A” or “R” include, for instance, alkyl, cycloalkyl, aryl, aralkyl, alkoxy, halogen, ether, thioether, disulphide, sulfoxide, sulfone, sulfonate, amino, aldehyde, keto, carboxylic acid ester, carboxylic acid, carbonate, carboxylate, cyano, alkylsilane and alkoxysilane groups, carboxylamide groups, and so forth.

The total number of recurring units of general formula (I) or formula (II) or of general formulae (I) and (II) is typically from 2 to 2,000, and in some embodiments, from 2 to 100.

Particularly suitable substituted polythiophenes are those in which “A” is an optionally substituted C₂ to C₃ alkylene radical and x is 0 or 1. In one particular embodiment, the substituted polythiophene is poly(3,4-ethylenedioxythiophene) (“PEDT”), which has recurring units of formula (II), wherein “A” is CH₂—CH₂ and “x” is 0. The monomers used to form such polymers may vary as desired. For instance, particularly suitable monomers are substituted 3,4-alkylenedioxythiophenes having the general formula (III), (IV), or both:

wherein, A, R, and X are as defined above.

Examples of such monomers include, for instance, optionally substituted 3,4-ethylenedioxythiophenes. One commercially suitable example of 3,4-ethylenedioxthiophene is available from H.C. Starck GmbH under the designation Clevios™ M. Derivatives of these monomers may also be employed that are, for example, dimers or trimers of the above monomers. Higher molecular derivatives, i.e., tetramers, pentamers, etc. of the monomers are suitable for use in the present invention. The derivatives may be made up of identical or different monomer units and used in pure form and in a mixture with one another and/or with the monomers. Oxidized or reduced forms of these precursors may also be employed.

The thiophene monomers, such as described above, are chemically polymerized in the presence of an oxidative catalyst. The oxidative catalyst typically includes a transition metal cation, such as iron(III), copper(II), chromium(VI), cerium(IV), manganese(IV), manganese(VII), ruthenium(III) cations, etc. A dopant may also be employed to provide excess charge to the conductive polymer and stabilize the conductivity of the polymer. The dopant typically includes an inorganic or organic anion, such as an ion of a sulfonic acid. In certain embodiments, the oxidative catalyst employed in the precursor solution has both a catalytic and doping functionality in that it includes a cation (e.g., transition metal) and anion (e.g., sulfonic acid). For example, the oxidative catalyst may be a transition metal salt that includes iron(III) cations, such as iron(III) halides (e.g., FeCl₃) or iron(III) salts of other inorganic acids, such as Fe(ClO₄)₃ or Fe₂(SO₄)₃ and the iron(III) salts of organic acids and inorganic acids comprising organic radicals. Examples of iron (III) salts of inorganic acids with organic radicals include, for instance, iron(III) salts of sulfuric acid monoesters of C₁ to C₂₀ alkanols (e.g., iron(III) salt of lauryl sulfate). Likewise, examples of iron(III) salts of organic acids include, for instance, iron(III) salts of C₁ to C₂₀ alkane sulfonic acids (e.g., methane, ethane, propane, butane, or dodecane sulfonic acid); iron (III) salts of aliphatic perfluorosulfonic acids (e.g., trifluoromethane sulfonic acid, perfluorobutane sulfonic acid, or perfluorooctane sulfonic acid); iron (III) salts of aliphatic C₁ to C₂₀ carboxylic acids (e.g., 2-ethylhexylcarboxylic acid); iron (III) salts of aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acid or perfluorooctane acid); iron (III) salts of aromatic sulfonic acids optionally substituted by C₁ to C₂₀ alkyl groups (e.g., benzene sulfonic acid, o-toluene sulfonic acid, p-toluene sulfonic acid, or dodecylbenzene sulfonic acid); iron (III) salts of cycloalkane sulfonic acids (e.g., camphor sulfonic acid); and so forth. Mixtures of these above-mentioned iron(III) salts may also be used. Iron(III)-p-toluene sulfonate, iron(III)-o-toluene sulfonate, and mixtures thereof, are particularly suitable. One commercially suitable example of iron(III)-p-toluene sulfonate is available from H.C. Starck GmbH under the designation Clevios™ C.

Prior to application to the metal substrate, a precursor solution is formed that contains both the monomeric thiophene and the oxidative catalyst. Among other things, this allows polymer chains to grow immediately adjacent to the surface of the metal substrate. To minimize the likelihood of premature polymerization, the oxidative catalyst is employed in an amount less than what is required to fully react all of the reagent assuming 100% yield (i.e., “stoichiometric amount”). This can slow the polymerization of the monomer, creating oligomers that are shorter than if fully polymerized, which can better penetrate into the roughened regions of the metal substrate. For example, the stoichiometric amount of the catalyst can be about 2 moles catalyst per 1 mole of monomer. In such cases, the oxidative catalyst may be employed in the present invention in an amount of less than about 2 moles of catalyst per 1 mole of monomer, in some embodiments less than about 1.5 moles of catalyst per 1 mole of monomer, in some embodiments from about 0.4 to about 1.5 moles of catalyst per 1 mole of monomer, and in some embodiments, from about 0.8 to about 1.4 moles of catalyst per 1 mole of monomer. The ratio of the weight of catalyst to the weight of monomer may likewise be less than about 20, in some embodiments less than about 15, in some embodiments from about 4 to about 15, and in some embodiments, from about 8 to about 14.

Premature polymerization may also be inhibited through the use of a reaction inhibitor. In one particular embodiment, for example, a polar aprotic solvent is employed as a reaction inhibitor. The localized charge on the aprotic solvent can attract, through electron donation (e.g., acid-base reactions), the positively charged metal (e.g., iron) of the oxidative catalyst to form a weakly bonded complex. This weak complex can inhibit the ability of the catalyst to oxidize the monomer for polymerization. As such, only a relatively small amount, if any, of the monomer is prematurely polymerized in the precursor solution prior to its application to the metal substrate. Additionally, such solvents can also dissolve oligomers that prematurely form while still in the precursor solution. Thus, the oligomers can be inhibited from further polymerization. Particularly suitable polar aprotic solvents include, but are not limited to, N-methylpyrrolidone, dimethyl sulfoxide, dimethylformamide, acetonitrile, hexamethylphosphorotriamide, dimethyl acetamide, methyl ethyl ketone, acetone, ethyl acetate, tetrahydrofuran, dichloromethane, and so forth.

In addition to aprotic solvents, other solvents (e.g., polar protic or non-polar) may also be employed in the solution, either alone or as a co-solvent with the aprotic solvent. Examples of suitable solvents include, for instance, glycols (e.g., propylene glycol, butylene glycol, triethylene glycol, hexylene glycol, polyethylene glycols, ethoxydiglycol, and dipropyleneglycol); glycol ethers (e.g., methyl glycol ether, ethyl glycol ether, and isopropyl glycol ether); ethers (e.g., diethyl ether and tetrahydrofuran); alcohols (e.g., methanol, ethanol, n-propanol, iso-propanol, and butanol); triglycerides; ketones; esters (e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate, and methoxypropyl acetate); amides (e.g., dimethylformamide, dimethylacetamide, dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones); nitriles (e.g., acetonitrile, propionitrile, butyronitrile and benzonitrile); sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane); and so forth. Particularly suitable solvents are aliphatic alcohols, such as ethanol, propanol, methanol, isopropanol, butanol, and so forth.

The total amount of solvents in the precursor solution may be from about 40 wt. % wt. % to about 90 wt. %, in some embodiments from about 50 wt. % to about 85 wt. %, and in some embodiments, from about 60 wt. % to about 80 wt. %. For instance, polar protic and/or non-polar solvents typically constitute from about 30 wt. % to about 90 wt. %, in some embodiments, and in some embodiments, from about 50 wt. % to about 80 wt. % of the precursor solution. Likewise, when employed, polar aprotic solvents typically constitute from about 0.001 wt. % to about 10 wt. %, and in some embodiments, from about 0.05 wt. % to about 1 wt. % of the precursor solution. The oxidative catalysts may also be employed in an amount from about 5 wt. % to about 50 wt. %, in some embodiments from about 10 to about 40 wt. %, and in some embodiments, from about 15 to about 30 wt. %, of the precursor solution, while the monomers may constitute from about 0.1 wt. % to about 10 wt. %, in some embodiments from about 0.5 wt. % to about 8 wt. %, and in some embodiments, from about 1 wt. % to about 6 wt. %, of the precursor solution

Once the solution is formed, it may be applied to the metal substrate using any known technique. For instance, suitable techniques may include dipping, spin coating, impregnation, pouring, dropwise application, injection, spraying, doctor blading, brushing or printing (e.g., ink-jet, screen, or pad printing). For example, the metal substrate may be dipped into or sprayed with the precursor solution. Although it may vary depending on the application technique employed, the viscosity of the solution is typically from about 0.1 to about 100,000 mPas (measured at a shear rate of 100 s⁻¹), in some embodiments from about 1 to about 10,000 mPas, in some embodiments from about 10 to about 1,500 mPas, and in some embodiments, from about 100 to about 1000 mPas.

Prior to and/or during application to the metal substrate, the temperature of the precursor solution may be selectively controlled to help stabilize the solution and inhibit premature polymerization. For example, the precursor solution may be maintained at a temperature of less than about 20° C., in some embodiments from about −10° C. to about 15° C., and in some embodiments, from about 0° C. to about 10° C. Once applied to the surface of the substrate, its temperature may be raised, such as to room temperature (e.g., 25° C.) or higher, in some embodiments from about 25° C. to about 250° C., to allow polymerization to proceed and form a conductive polymer coating thereon. If desired, the above-described application steps may be repeated until the desired thickness of the coating is achieved. In some embodiments, only a relatively thin layer of the coating is formed at a time. The total target thickness of the coating may generally vary depending on the desired properties of the capacitor. Typically, the resulting conductive polymer coating has a thickness of from about 0.2 micrometers (“μm”) to about 50 μm, in some embodiments from about 0.5 μm to about 20 μm, and in some embodiments, from about 1 μm to about 5 μm. It should be understood that the thickness of the coating is not necessarily the same at all locations on the metal substrate. Nevertheless, the average thickness of the coating on the substrate generally falls within the ranges noted above.

The conductive polymer coating may optionally be healed. Healing may occur after each application of a conductive polymer layer or may occur after the application of the entire conductive polymer coating. In some embodiments, the conductive polymer can be healed by dipping the metal substrate into an electrolyte solution, and thereafter applying a constant voltage to the solution until the current is reduced to a preselected level. If desired, such healing can be accomplished in multiple steps. For example, an electrolyte solution can be a dilute solution of the monomer, the catalyst, and dopant in an alcohol solvent (e.g., ethanol). The coating may also be washed if desired to remove various byproducts, excess reagents, and so forth.

Without intending to be limited by theory, it is believed that charging of the capacitor to a high voltage (e.g., greater than the formation voltage) forces ions of the electrolyte into coatings containing such substituted polythiophenes. This causes the conductive polymer to “swell” and retain the ions near the surface, thereby enhancing charge density. Because the polymer is generally amorphous and non-crystalline, it can also dissipate and/or absorb the heat associated with the high voltage. Upon discharge, it is also believed that the substituted polythiophene “relaxes” and allows ions in the electrolyte to move out of the coating. Through such swelling and relaxation mechanism, charge density near the metal substrate can be increased without a chemical reaction with the electrolyte. Accordingly, one beneficial aspect of the present invention is that mechanical robustness and good electrical performance may be provided without the need for conventional conductive coatings, such as those made from activated carbon or metal oxides (e.g., ruthenium oxide). In fact, the present inventors have discovered that excellent results may be achieved using the coating as the principal material on the metal substrate. That is, the coating may constitute at least about 90 wt. %, in some embodiments at least about 92 wt. %, and in some embodiments, at least about 95 wt. % of the material(s) present on the metal substrate. Nevertheless, it should be understood that other conductive coatings may also be used in some embodiments of the present invention.

II. Anode

The anode of the electrolytic capacitor includes a porous body that may be formed from a valve metal composition. The specific charge of the composition may vary. In certain embodiments, for example, compositions having a high specific charge are employed, such as about 5,000 μF*V/g or more, in some embodiments about 25,000 μF*V/g or more, in some embodiments about 40,000 μF*V/g or more, and in some embodiments, from about 70,000 to about 300,000 μF*V/g. The valve metal composition contains a valve metal (i.e., metal that is capable of oxidation) or valve metal-based compound, such as tantalum, niobium, aluminum, hafnium, titanium, alloys thereof, oxides thereof, nitrides thereof, and so forth. For example, the valve metal composition may contain an electrically conductive oxide of niobium, such as niobium oxide having an atomic ratio of niobium to oxygen of 1:1.0±1.0, in some embodiments 1:1.0±0.3, in some embodiments 1:1.0±0.1, and in some embodiments, 1:1.0±0.05. For example, the niobium oxide may be NbO_(0.7), NbO_(1.0), NbO_(1.1), and NbO₂. Examples of such valve metal oxides are described in U.S. Pat. Nos. 6,322,912 to Fife; 6,391,275 to Fife et al.; 6,416,730 to Fife et al.; 6,527,937 to Fife; 6,576,099 to Kimmel, et al.; 6,592,740 to Fife, et al.; and 6,639,787 to Kimmel, et al.; and 7,220,397 to Kimmel, et al., as well as U.S. Patent Application Publication Nos. 2005/0019581 to Schnitter; 2005/0103638 to Schnitter, et al.; 2005/0013765 to Thomas, et al., all of which are incorporated herein in their entirety by reference thereto for all purposes.

Conventional fabricating procedures may generally be utilized to form the porous anode body. In one embodiment, a tantalum or niobium oxide powder having a certain particle size is first selected. The particles may be flaked, angular, nodular, and mixtures or variations thereof. The particles also typically have a screen size distribution of at least about 60 mesh, in some embodiments from about 60 to about 325 mesh, and in some embodiments, from about 100 to about 200 mesh. Further, the specific surface area is from about 0.1 to about 10.0 m²/g, in some embodiments from about 0.5 to about 5.0 m²/g, and in some embodiments, from about 1.0 to about 2.0 m²/g. The term “specific surface area” refers to the surface area determined by the physical gas adsorption (B.E.T.) method of Bruanauer, Emmet, and Teller, Journal of American Chemical Society, Vol. 60, 1938, p. 309, with nitrogen as the adsorption gas. Likewise, the bulk (or Scott) density is typically from about 0.1 to about 5.0 g/cm³, in some embodiments from about 0.2 to about 4.0 g/cm³, and in some embodiments, from about 0.5 to about 3.0 g/cm³.

To facilitate the construction of the anode body, other components may be added to the electrically conductive particles. For example, the electrically conductive particles may be optionally mixed with a binder and/or lubricant to ensure that the particles adequately adhere to each other when pressed to form the anode body. Suitable binders may include camphor, stearic and other soapy fatty acids, Carbowax (Union Carbide), Glyptal (General Electric), polyvinyl alcohols, naphthalene, vegetable wax, and microwaxes (purified paraffins). The binder may be dissolved and dispersed in a solvent. Exemplary solvents may include water, alcohols, and so forth. When utilized, the percentage of binders and/or lubricants may vary from about 0.1% to about 8% by weight of the total mass. It should be understood, however, that binders and lubricants are not required in the present invention.

The resulting powder may be compacted using any conventional powder press mold. For example, the press mold may be a single station compaction press using a die and one or multiple punches. Alternatively, anvil-type compaction press molds may be used that use only a die and single lower punch. Single station compaction press molds are available in several basic types, such as cam, toggle/knuckle and eccentric/crank presses with varying capabilities, such as single action, double action, floating die, movable platen, opposed ram, screw, impact, hot pressing, coining or sizing. If desired, any binder/lubricant may be removed after compression, such as by heating the formed pellet under vacuum at a certain temperature (e.g., from about 150° C. to about 500° C.) for several minutes. Alternatively, the binder/lubricant may also be removed by contacting the pellet with an aqueous solution, such as described in U.S. Pat. No. 6,197,252 to Bishop, et al., which is incorporated herein in its entirety by reference thereto for all purposes.

The size of the pressed anode body may depend in part on the desired size of the metal substrate. In certain embodiments, the length of the anode body may range from about 1 to about 100 millimeters, in some embodiments from about 5 to about 60 millimeters, and in some embodiments, from about 5 to about 20 millimeters. The width (or diameter) of the anode body may also range from about 0.5 to about 20 millimeters, in some embodiments from about 1 to about 20 millimeters, and in some embodiments, from about 4 to about 10 millimeters. The shape of the anode body may also be selected to improve the electrical properties of the resulting capacitor. For example, the anode body may have a shape that is cylindrical, rectangular, D-shaped, curved, etc.

The anode body may be anodically oxidized (“anodized”) so that a dielectric layer is formed over and/or within the anode. For example, a tantalum (Ta) anode may be anodized to tantalum pentoxide (Ta₂O₅). Typically, anodization is performed by initially applying a solution to the anode, such as by dipping anode into the electrolyte. A solvent is generally employed, such as water (e.g., deionized water). To enhance ionic conductivity, a compound may be employed that is capable of dissociating in the solvent to form ions. Examples of such compounds include, for instance, acids, such as described below with respect to the electrolyte. For example, an acid (e.g., phosphoric acid) may constitute from about 0.01 wt. % to about 5 wt. %, in some embodiments from about 0.05 wt. % to about 0.8 wt. %, and in some embodiments, from about 0.1 wt. % to about 0.5 wt. % of the anodizing solution. If desired, blends of acids may also be employed.

A current is passed through the anodizing solution to form the dielectric layer. The value of the formation voltage manages the thickness of the dielectric layer. For example, the power supply may be initially set up at a galvanostatic mode until the required voltage is reached. Thereafter, the power supply may be switched to a potentiostatic mode to ensure that the desired dielectric thickness is formed over the entire surface of the anode. Of course, other known methods may also be employed, such as pulse or step potentiostatic methods. The voltage at which anodic oxidation occurs typically ranges from about 4 to about 250 V, and in some embodiments, from about 9 to about 200 V, and in some embodiments, from about 20 to about 150 V. During oxidation, the anodizing solution can be kept at an elevated temperature, such as about 30° C. or more, in some embodiments from about 40° C. to about 200° C., and in some embodiments, from about 50° C. to about 100° C. Anodic oxidation can also be done at ambient temperature or lower. The resulting dielectric layer may be formed on a surface of the anode and within its pores.

III. Electrolyte

The electrolyte is the electrically active material that provides the connecting path between the anode and cathode. Various suitable electrolytes are described in U.S. Pat. Nos. 5,369,547 and 6,594,140 to Evans, et al., which are incorporated herein their entirety by reference thereto for all purposes. Typically, the electrolyte is ionically conductive in that has an ionic conductivity of from about 0.5 to about 1000 milliSiemens per centimeter (“mS/cm”), in some embodiments from about 1 to about 100 mS/cm, in some embodiments from about 5 mS/cm to about 100 mS/cm, and in some embodiments, from about 10 to about 50 mS/cm, determined at a temperature of 25° C. using any known electric conductivity meter (e.g., Oakton Con Series 11). Within the ranges noted above, it is believed that the ionic conductivity of the electrolyte allows the electric field to extend into the electrolyte to a length (Debye length) sufficient to result in significant charge separation. This extends the potential energy of the dielectric to the electrolyte so that the resulting capacitor is able to store even more potential energy than predicted by the thickness of the dielectric. In other words, the capacitor may be charged to a voltage that exceeds the formation voltage of the dielectric. The ratio of the voltage to which the capacitor can be charged to the formation voltage may, for instance, be from about 1.0 to 2.0, in some embodiments from about 1.1 to about 1.8, and in some embodiments, from about 1.2 to about 1.6. As an example, the voltage to which the capacitor is charged may be from about 200 to about 350 V, in some embodiments from about 220 to about 320 V, and in some embodiments, from about 250 to about 300V.

The electrolyte is generally in the form of a liquid, such as a solution (e.g., aqueous or non-aqueous), precursor solution, gel, etc. For example, the working electrolyte may be an aqueous solution of an acid (e.g., sulfuric acid, phosphoric acid, or nitric acid), base (e.g., potassium hydroxide), or salt (e.g., ammonium salt, such as a nitrate), as well any other suitable electrolyte known in the art, such as a salt dissolved in an organic solvent (e.g., ammonium salt dissolved in a glycol-based solution). Various other electrolytes are described in U.S. Pat. Nos. 5,369,547 and 6,594,140 to Evans, et al., which are incorporated herein their entirety by reference thereto for all purposes.

The desired ionic conductivity may be achieved by selecting ionic compound(s) (e.g., acids, bases, salts, and so forth) within certain concentration ranges. In one particular embodiment, salts of weak organic acids may be effective in achieving the desired conductivity of the electrolyte. The cation of the salt may include monatomic cations, such as alkali metals (e.g., Li⁺, Na⁺, K⁺, Rb⁺, or Cs⁺), alkaline earth metals (e.g., Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺), transition metals (e.g., Ag⁺, Fe²⁺, Fe³⁺, etc.), as well as polyatomic cations, such as NH₄ ⁺. The monovalent ammonium (NH₄ ⁺), sodium (K⁺), and lithium (Li⁺) are particularly suitable cations for use in the present invention. The organic acid used to form the anion of the salt is “weak” in the sense that it typically has a first acid dissociation constant (pK_(a1)) of about 0 to about 11, in some embodiments about 1 to about 10, and in some embodiments, from about 2 to about 10, determined at 25° C. Any suitable weak organic acids may be used in the present invention, such as carboxylic acids, such as acrylic acid, methacrylic acid, malonic acid, succinic acid, salicylic acid, sulfosalicylic acid, adipic acid, maleic acid, malic acid, oleic acid, gallic acid, tartaric acid (e.g., dextotartaric acid, mesotartaric acid, etc.), citric acid, formic acid, acetic acid, glycolic acid, oxalic acid, propionic acid, phthalic acid, isophthalic acid, glutaric acid, gluconic acid, lactic acid, aspartic acid, glutaminic acid, itaconic acid, trifluoroacetic acid, barbituric acid, cinnamic acid, benzoic acid, 4-hydroxybenzoic acid, aminobenzoic acid, etc.; blends thereof, and so forth. Polyprotic acids (e.g., diprotic, triprotic, etc.) are particularly desirable for use in forming the salt, such as adipic acid (pK_(a1) of 4.43 and pK_(a2) of 5.41), α-tartaric acid (pK_(a1) of 2.98 and pK_(a2) of 4.34), meso-tartaric acid (pK_(a1) of 3.22 and pK_(a2) of 4.82), oxalic acid (pK_(a1) of 1.23 and pK_(a2) of 4.19), lactic acid (pK_(a1) of 3.13, pK_(a2) of 4.76, and pK_(a3) of 6.40), etc.

While the actual amounts may vary depending on the particular salt employed, its solubility in the solvent(s) used in the electrolyte, and the presence of other components, such weak organic acid salts are typically present in the electrolyte in an amount of from about 0.1 to about 25 wt. %, in some embodiments from about 0.2 to about 20 wt. %, in some embodiments from about 0.3 to about 15 wt. %, and in some embodiments, from about 0.5 to about 5 wt. %.

The electrolyte is typically aqueous in that it contains an aqueous solvent, such as water (e.g., deionized water). For example, water (e.g., deionized water) may constitute from about 20 wt. % to about 95 wt. %, in some embodiments from about 30 wt. % to about 90 wt. %, and in some embodiments, from about 40 wt. % to about 85 wt. % of the electrolyte. A secondary solvent may also be employed to form a solvent mixture. Suitable secondary solvents may include, for instance, glycols (e.g., ethylene glycol, propylene glycol, butylene glycol, triethylene glycol, hexylene glycol, polyethylene glycols, ethoxydiglycol, dipropyleneglycol, etc.); glycol ethers (e.g., methyl glycol ether, ethyl glycol ether, isopropyl glycol ether, etc.); alcohols (e.g., methanol, ethanol, n-propanol, iso-propanol, and butanol); ketones (e.g., acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters (e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate, methoxypropyl acetate, ethylene carbonate, propylene carbonate, etc.); amides (e.g., dimethylformamide, dimethylacetamide, dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones); sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane); and so forth. Such solvent mixtures typically contain water in an amount from about 40 wt. % to about 80 wt. %, in some embodiments from about 50 wt. % to about 75 wt. %, and in some embodiments, from about 55 wt. % to about 70 wt. % and secondary solvent(s) in an amount from about 20 wt. % to about 60 wt. %, in some embodiments from about 25 wt. % to about 50 wt. %, and in some embodiments, from about 30 wt. % to about 45 wt. %. The secondary solvent(s) may, for example, constitute from about 5 wt. % to about 45 wt. %, in some embodiments from about 10 wt. % to about 40 wt. %, and in some embodiments, from about 15 wt. % to about 35 wt. % of the electrolyte.

If desired, the electrolyte may be relatively neutral and have a pH of from about 4.5 to about 7.0, in some embodiments from about 5.0 to about 6.5, and in some embodiments, from about 5.5 to about 6.0. One or more pH adjusters (e.g., acids, bases, etc.) may be employed to help achieve the desired pH. In one embodiment, an acid is employed to lower the pH to the desired range. Suitable acids include, for instance, inorganic acids, such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, polyphosphoric acid, boric acid, boronic acid, etc.; organic acids, including carboxylic acids, such as acrylic acid, methacrylic acid, malonic acid, succinic acid, salicylic acid, sulfosalicylic acid, adipic acid, maleic acid, malic acid, oleic acid, gallic acid, tartaric acid, citric acid, formic acid, acetic acid, glycolic acid, oxalic acid, propionic acid, phthalic acid, isophthalic acid, glutaric acid, gluconic acid, lactic acid, aspartic acid, glutaminic acid, itaconic acid, trifluoroacetic acid, barbituric acid, cinnamic acid, benzoic acid, 4-hydroxybenzoic acid, aminobenzoic acid, etc.; sulfonic acids, such as methanesulfonic acid, benzenesulfonic acid, toluenesulfonic acid, trifluoromethanesulfonic acid, styrenesulfonic acid, naphthalene disulfonic acid, hydroxybenzenesulfonic acid, etc.; polymeric acids, such as poly(acrylic) or poly(methacrylic) acid and copolymers thereof (e.g., maleic-acrylic, sulfonic-acrylic, and styrene-acrylic copolymers), carageenic acid, carboxymethyl cellulose, alginic acid, etc.; and so forth. Although the total concentration of pH adjusters may vary, they are typically present in an amount of from about 0.01 wt. % to about 10 wt. %, in some embodiments from about 0.05 wt. % to about 5 wt. %, and in some embodiments, from about 0.1 wt. % to about 2 wt % of the electrolyte.

The electrolyte may also contain other components that help improve the electrical performance of the capacitor. For instance, a depolarizer may be employed in the electrolyte to help inhibit the evolution of hydrogen gas at the cathode of the electrolytic capacitor, which could otherwise cause the capacitor to bulge and eventually fail. When employed, the depolarizer normally constitutes from about 1 to about 500 parts per million (“ppm”), in some embodiments from about 10 to about 200 ppm, and in some embodiments, from about 20 to about 150 ppm of the electrolyte. Suitable depolarizers may include nitroaromatic compounds, such as 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2-nitrobenzonic acid, 3-nitrobenzonic acid, 4-nitrobenzonic acid, 2-nitroacetophenone, 3-nitroacetophenone, 4-nitroacetophenone, 2-nitroanisole, 3-nitroanisole, 4-nitroanisole, 2-nitrobenzaldehyde, 3-nitrobenzaldehyde, 4-nitrobenzaldehyde, 2-nitrobenzyl alcohol, 3-nitrobenzyl alcohol, 4-nitrobenzyl alcohol, 2-nitrophthalic acid, 3-nitrophthalic acid, 4-nitrophthalic acid, and so forth. Particularly suitable nitroaromatic depolarizers for use in the present invention are nitrobenzoic acids, anhydrides or salts thereof, substituted with one or more alkyl groups (e.g., methyl, ethyl, propyl, butyl, etc). Specific examples of such alkyl-substituted nitrobenzoic compounds include, for instance, 2-methyl-3-nitrobenzoic acid; 2-methyl-6-nitrobenzoic acid; 3-methyl-2-nitrobenzoic acid; 3-methyl-4-nitrobenzoic acid; 3-methyl-6-nitrobenzoic acid; 4-methyl-3-nitrobenzoic acid; anhydrides or salts thereof; and so forth. Without intending to be limited by theory, it is believed that alkyl-substituted nitrobenzoic compounds may be preferentially electrochemically adsorbed on the active sites of the cathode surface when the cathode potential reaches a low region or the cell voltage is high, and may be subsequently desorbed therefrom into the electrolyte when the cathode potential goes up or the cell voltage is low. In this manner, the compounds are “electrochemically reversible”, which may provide improved inhibition of hydrogen gas production.

The particular manner in which the components are incorporated into the capacitor is not critical and may be accomplished using a variety of techniques. Referring to FIG. 1, for example, one embodiment of an electrolytic capacitor 40 is shown that includes an electrolyte 44 disposed in electrical contact with an anode 20 and cathode 43. The anode 20 contains a dielectric layer (not shown) and is in electrical contact with a lead 42. The lead 42 may be formed from any electrically conductive material, such as tantalum, niobium, nickel, aluminum, hafnium, titanium, etc., as well as oxides and/or nitrides of thereof. In certain embodiments, electrical contact with the anode 20 may be accomplished by electrically coupling the lead 42 by resistance or laser welding. The cathode 43 is formed from a metal substrate 41, such as described above, and a conductive polymer coating 49. In this embodiment, the cathode substrate 41 is in the form of a cylindrically-shaped “can” with an attached lid. As shown, the conductive polymer coating 49 in this embodiment is formed on an interior surface of the substrate 41.

A liquid seal 23 (e.g., glass-to-metal) may be employed that connects and seals the anode 20 to the cathode 43. An electrically insulating bushing 89 (e.g., polytetrafluoroethylene (“PTFE”)) and/or support 91 may also be employed to help stabilize the anode 20 and lead 42 and maintain the desired spacing within the capacitor. If desired, a separator (not shown) may also be positioned between the cathode 43 and anode 20 to prevent direct contact between the anode and cathode, yet permit ionic current flow of the electrolyte 44 to the electrodes. Examples of suitable materials for this purpose include, for instance, porous polymer materials (e.g., polypropylene, polyethylene, polycarbonate, etc.), porous inorganic materials (e.g., fiberglass mats, porous glass paper, etc.), ion exchange resin materials, etc. Particular examples include ionic perfluoronated sulfonic acid polymer membranes (e.g., Nafion™ from the E.I. DuPont de Nemeours & Co.), sulphonated fluorocarbon polymer membranes, polybenzimidazole (PBI) membranes, and polyether ether ketone (PEEK) membranes. Although preventing direct contact between the anode and cathode, the separator permits ionic current flow of the electrolyte to the electrodes.

Regardless of its particular configuration, the capacitor of the present invention may exhibit excellent electrical properties. For example, due in part to high conductivity, the capacitor of the present invention can achieve excellent electrical properties and thus be suitable for use in the capacitor bank of the implantable medical device. For example, the equivalent series resistance (“ESR”)—the extent that the capacitor acts like a resistor when charging and discharging in an electronic circuit—may be less than about 1500 milliohms, in some embodiments less than about 1000 milliohms, and in some embodiments, less than about 500 milliohms, measured with a 2-volt bias and 1-volt signal at a frequency of 120 Hz. Likewise, the capacitance may be about 1 milliFarad per square centimeter (“mF/cm²”) or more, in some embodiments about 2 mF/cm² or more, in some embodiments from about 5 to about 50 mF/cm², and in some embodiments, from about 8 to about 20 mF/cm².

The electrolytic capacitor of the present invention may be used in various applications, including but not limited to medical devices, such as implantable defibrillators, pacemakers, cardioverters, neural stimulators, drug administering devices, etc.; automotive applications; military applications, such as RADAR systems; consumer electronics, such as radios, televisions, etc.; and so forth. In one embodiment, for example, the capacitor may be employed in an implantable medical device configured to provide a therapeutic high voltage (e.g., between approximately 500 Volts and approximately 850 Volts, or, desirably, between approximately 600 Volts and approximately 900 Volts) treatment for a patient. The device may contain a container or housing that is hermetically sealed and biologically inert. One or more leads are electrically coupled between the device and the patient's heart via a vein. Cardiac electrodes are provided to sense cardiac activity and/or provide a voltage to the heart. At least a portion of the leads (e.g., an end portion of the leads) may be provided adjacent or in contact with one or more of a ventricle and an atrium of the heart. The device also contains a capacitor bank that typically contains two or more capacitors connected in series and coupled to a battery that is internal or external to the device and supplies energy to the capacitor bank. Due in part to high conductivity, the capacitor of the present invention can achieve excellent electrical properties and thus be suitable for use in the capacitor bank of the implantable medical device.

The present invention may be better understood by reference to the following examples.

Test Procedures

All test procedures were measured in conjunction with a cylindrical tantalum anode pressed to a size of 17.3 mm (length)×7.2 mm (diameter) and weight of 4.4 g, and anodized to 10V. The anode exhibited a capacitance of 6.8 mF at a frequency of 120 Hz. The electrolyte was a 5.0 M aqueous solution of sulfuric acid (specific gravity of 1.26 g/cm³). The wet capacitance was determined from following formula:

1/C _(wet)=1/C _(anode)+1C _(cathode)

Equivalent Series Resistance (ESR)

Equivalence series resistance may be measured using a Keithley 3330 Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 volt peak to peak sinusoidal signal. The operating frequency was 120 Hz and the temperature was 23° C.±2° C.

Wet Capacitance

The capacitance was measured using a Keithley 3330 Precision LCZ meter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak to peak sinusoidal signal. The operating frequency was 120 Hz and the temperature was 23° C.±2° C.

Storage Test

Certain electrical properties (ESR and capacitance) were determined after storage testing. More particularly, 10 samples were filled with 5.0 M aqueous solution of sulfuric acid (specific gravity of 1.26 g/cm³) for 250 hours at ambient temperature. The samples were then tested in the manner described above.

Temperature+Pressure Test:

Certain electrical properties (ESR and capacitance) were determined after temperature and pressure testing. More particularly, 10 samples were put into a pressure cooker filled with 5.0 M aqueous solution of sulfuric acid (specific gravity of 1.26 g/cm³) for 100 hours at 125° C. The samples were then tested in the manner described above.

Example 1

Initially, 100 pieces of cylindrical tantalum cans with a size of 18.3 mm (length)×9.1 mm (internal diameter) were sandblasted with a JetStreem Blaster II (SCM System, inc.). The sandblasting media was black silicone carbide grit having a size of 63 to 106 μm). The media flow rate was 0.5 grams per second via a 3.2-millimeter blasting nozzle. All pieces of cylindrical tantalum cans were sandblasted to a control level 10.7 millimeters (from potential 18.3 millimeters), using appropriate ferrules. The sandblasting time was 20 seconds. These samples were then degreased in water with surfactants in an ultrasonic bath for 5 minutes, rinsed 3 times in deionized water, and then dried at a temperature of 85° C. for 5 minutes. A precursor solution was thereafter applied to the microroughened surface that contained four (4) parts by weight of ethanol (Sigma-Aldrich, Co.), 0.1 part by weight of methylpyrrolidone (Sigma-Aldrich, Co.), 1 part by weight of 3,4-ethylenedioxythiophene (H.C. Starck GmbH under the designation Clevios™ M), and 10 parts by weight of 40% butanol solution of iron(III)-p-toluene sulfonate (H.C. Starck GmbH under the designation Clevios™ C). The tantalum cans were filled to the control level with the polymerization precursor solution for five (5) minutes. The cans were then drained using a vacuum for up to one (1) minute, and were then put into a drying oven for 15 minutes at 85° C. The resulting structure of poly(3,4-ethylenedioxythiophene) was washed in methanol to remove reaction by-products for five (5) minutes and the tantalum cans were put into a drying oven for five (5) minutes at 85° C. This polymerization cycle was repeated four (4) times.

Example 2

100 pieces of cylindrical tantalum cans were prepared as described in Example 1, except that the sandblasting time was 15 seconds.

Example 3

100 pieces of cylindrical tantalum cans were prepared as described in Example 1, except that the sandblasting time was 10 seconds.

Example 4

100 pieces of cylindrical tantalum cans were prepared as described in Example 1, except that the sandblasting time was 5 seconds.

Example 5

100 pieces of cylindrical tantalum cans were sandblasted as described in Example 1. Thereafter, a conductive polymer coating was formed by dipping the cans into a butanol solution of iron (III) toluenesulfonate (Clevios™ C, H.C. Starck) for five (5) minutes and subsequently into 3,4-ethylenedioxythiophene (Clevios™ M, H.C. Starck) for five (5) minutes. The cans were drained using a vacuum for up to one (1) minute and were put into a drying oven for 45 minutes at 30° C. The resulting structure of poly(3,4-ethylenedioxythiophene) was washed in methanol to remove reaction by-products for five (5) minutes and the tantalum cans were put into a drying oven for five (5) minutes at 85° C. This polymerization cycle was repeated (4) times.

Once formed, 10 samples of the cathodes of Examples 1-5 were then tested for capacitance and ESR in the manner described above. Furthermore, the capacitance and ESR of the samples was also measured after “storage testing” and “temperature/pressure testing” as described above. The results are shown below in Tables 1 and 2.

TABLE 1 Storage Testing Average (before Average Ex- testing) (after testing) am- CAP ESR CAP ESR ΔCAP ΔESR ΔCAP ΔESR ple (mF) (mΩ) (μF) (mΩ) (mF) (mΩ) [%] [%] 1 7.36 124 7.41 111 0.05 −13 0.7 −10.2 5 3.48 209 2.09 183 −1.39 −26 −39.9 −12.4

TABLE 2 Temperature/Pressure Testing Average (before Average Ex- testing) (after testing) am- CAP ESR CAP ESR ΔCAP ΔESR ΔCAP ΔESR ple (mF) (mΩ) (μF) (mΩ) (mF) (mΩ) [%] [%] 1 7.38 123 7.42 115 0.04 −9 0.5 −7.0 2 7.37 124 7.44 108 0.07 −15 1.0 −12.4 3 7.32 125 7.38 109 0.06 −16 0.9 −12.8 4 7.37 123 7.40 113 0.03 −11 0.4 −8.6 5 3.46 219 2.04 194 −1.42 −25 −41.0 −11.4

As indicated, the capacitance values of the sequentially applied conductive polymer coating (Example 5) decreased significantly after testing. In contrast, the capacitance values of the mixed monomer/catalyst system (Examples 1-4) actually experienced a slight increase. Without intending to be limited by theory, it is believed that the enhanced performance of Examples 1-4 is due in part to the excellent adhesion to the tantalum substrate exhibited by the mixed monomer/catalyst system.

These and other modifications and variations of the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims. 

What is claimed is:
 1. A method for forming a cathode of a wet electrolytic capacitor, the method comprising: applying a precursor solution to a roughened surface of a metal substrate, wherein the precursor solution contains a substituted thiophene monomer and an oxidative catalyst and the oxidative catalyst is employed in an amount less than the stoichiometric amount; and polymerizing the substituted thiophene monomer to form a coating that contains an intrinsically conductive substituted polythiophene.
 2. The method of claim 1, wherein the substituted polythiophene has recurring units of general formula (I), formula (II), or both:

wherein, A is an optionally substituted C₁ to C₅ alkylene radical; R is a linear or branched, optionally substituted C₁ to C₁₈ alkyl radical; optionally substituted C₅ to C₁₂ cycloalkyl radical; optionally substituted C₆ to C₁₄ aryl radical; optionally substituted C₇ to C₁₈ aralkyl radical; optionally substituted C₁ to C₄ hydroxyalkyl radical, or hydroxyl radical; and x is an integer from 0 to
 8. 3. The method of claim 2, wherein A is an optionally substituted C₂ to C₃ alkylene radical and x is 0 or
 1. 4. The method of claim 1, wherein the substituted polythiophene is poly(3,4-ethylenedioxythiophene).
 5. The method of claim 1, wherein the monomer includes a 3,4-alkylenedioxythiophene.
 6. The method of claim 5, wherein the monomer includes 3,4-ethylenedioxythiophene or a derivative thereof.
 7. The method of claim 1, wherein the oxidative catalyst includes an iron salt of an inorganic acid, an iron salt of an organic acid, or a combination thereof.
 8. The method of claim 7, wherein the oxidative catalyst includes iron(III)-p-toluene sulfonate, iron(III)-o-toluene sulfonate, or a combination thereof.
 9. The method of claim 1, wherein the precursor solution contains less than about 2 moles of the catalyst per mole of the monomer.
 10. The method of claim 1, wherein the precursor solution contains from about 0.8 to about 1.4 moles of the catalyst per mole of the monomer.
 11. The method of claim 1, wherein the precursor solution contains a polar protic solvent, non-polar solvent, aprotic solvent, or a combination thereof.
 12. The method of claim 1, wherein the solution is maintained at a temperature of from about −10° C. to about 15° C. prior to polymerization.
 13. The method of claim 1, wherein the metal substrate comprises titanium, tantalum, or a combination thereof.
 14. The method of claim 1, wherein the surface of the metal substrate is mechanically abraded.
 15. The method of claim 1, wherein the surface of the metal substrate is chemically etched, electrochemically etched, or a combination thereof.
 16. A wet electrolytic capacitor comprising an anodically oxidized anode, a liquid electrolyte, and a cathode formed according to the method of claim
 1. 17. A method for forming a wet electrolytic capacitor, the method comprising: applying a precursor solution to a roughened surface of a metal casing, wherein the precursor solution contains a substituted thiophene monomer and an oxidative catalyst and the oxidative catalyst is employed in an amount less than the stoichiometric amount; polymerizing the substituted thiophene monomer to form a coating that contains an intrinsically conductive substituted polythiophene; and placing the coated metal casing into electrical communication with an anode and electrolyte, wherein the anode is formed from a porous anode body that contains a dielectric layer.
 18. The method of claim 17, wherein the anode body includes tantalum, niobium, or an electrically conductive oxide thereof.
 19. The method of claim 17, wherein the liquid electrolyte is aqueous.
 20. The method of claim 17, wherein the liquid electrolyte has a pH of from about 4.5 to about 7.0.
 21. The method of claim 17, wherein the liquid electrolyte includes sulfuric acid.
 22. The method of claim 17, wherein the casing is generally cylindrical. 